Surfactants and blends of surfactants are important components of laundry detergents, dish detergents, household or industrial cleaners, personal care products, agricultural products, building materials, oil recovery compositions, emulsion polymers, and other products. Blends of surfactants are used frequently to achieve performance characteristics that are not easy to accomplish with a single surfactant type. For example, blends of anionic with nonionic or amphoteric surfactants are commonly used to formulate two-in-one shampoo and conditioner formulations for hair care.
When two surfactants used together provide unexpected surface characteristics compared with what could have been predicted based on summing their individual contributions, the combination exhibits synergy. Synergistic surfactant blends offer economic advantages because the benefits of each component can be realized at lower concentrations. Synergism has been quantified in mathematical terms. See, for example, the background discussion of U.S. Pat. No. 5,360,571, which explains the relationship between critical micelle concentration and βm, the mixed micelle parameter. As the reference explains, negative β values correspond to synergism, and more negative values indicate greater synergy.
Synergy can also be identified by measuring interfacial tension (IFT) as a function of surfactant blend composition. Minima in such plots correspond to blends having the highest synergy level. See, for example, U.S. Pat. No. 5,441,541 or 5,472,455, particularly Example 2 and FIG. 6. The '541 patent teaches that blends of certain cationic and anionic surfactants exhibit synergism, and the effect is maximized when equimolar amounts of the surfactants are used. According to the patentees, the “strong synergism in surface tension reduction effectiveness and efficiency implies the formation of a new active moiety” ('541 patent at Ex. 2).
Strong synergy has been observed in blends of cationic and anionic surfactants. However, the ability to form complexes and achieve a synergistic effect has tradeoffs, particularly with regard to solubility. Blends of cationic and anionic surfactants are often avoided because the complexes tend to precipitate, especially when the blends are diluted with water. According to U.S. Pat. No. 6,306,805, “most anionic-cationic surfactant mixtures studied are insoluble or only slightly soluble in water . . . At present, very few anionic-cationic surfactant mixtures have been found which produce clear solution phases over a wide concentration range at equimolar composition” ('805 patent at col. 3, II. 3-13). The reference acknowledges the high probability of synergism in mixtures of anionic and cationic surfactants, but qualifies its value: “However . . . the variations in surfactant type and size that produce progressively more negative β values unfortunately are accompanied by decreasing solubility. Hence, anionic-cationic synergism is limited by the formation of an insoluble salt, which typically occurs when the combined number of carbon atoms in the chains of both surfactants totals more than about twenty” ('805 patent at col. 3, II. 20-43). To overcome the solubility issue, the patentees use a ternary blend that includes a semi-polar nonionic, ethoxylated alkanolamide, or amphoteric/zwitterionic component as a “bridging surfactant.”
Improvements in metathesis catalysts (see J.C. Mol, Green Chem. 4 (2002) 5) have enabled the manufacture of reduced chain length, monounsaturated feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. We recently described how to synthesize a variety of valuable anionic and cationic surfactants from metathesis-based, monounsaturated feedstocks (see, e.g., copending PCT Int. Appl. Nos. US11/57595, US11/57596, US11/57597, US11/57602, US11/57605, and US11/57609, all filed 25 Oct. 2011). Among the cationic surfactants, for instance, we described quaternized fatty amines and quaternized fatty amidoamines made from metathesis-derived C10-C17 monounsaturated acids and their ester derivatives. Among the anionic surfactants, we described sulfonated esters, sulfoestolides, and fatty amide sulfonates made from metathesis-derived C10-C17 monounsaturated acids, octadecene-1,18-dioic acid, or their ester derivatives.
Given the tendency of combinations of anionic and cationic surfactants to precipitate from aqueous solutions, particularly when their combined carbon number exceeds twenty, it was unclear whether surfactants made from monounsaturated, metathesis-based feedstocks (with typical carbon numbers 10-18 for one portion of the complex) would offer any advantage for cationic-anionic surfactant blends, even if the blends happened to demonstrate synergy. However, the potential benefits of synergy invited us to explore this possibility.
In sum, the surfactant industry would benefit from the availability of new cationic-anionic surfactant blends, particularly blends that exhibit synergy and could be used to improve the performance and/or economics of end-use applications. Valuable blends would take advantage of the now-available, methathesis-based feedstocks based on soybean oil, palm oil, or other renewable resources. Ideally, the blends would avoid the solubility issues that have, until now, limited their applicability.